Do not add _1 to file name when only one screenshot is produced

This is about taking a single screenshot from a video.

The documentation suggests that a _1 is only appended to the file name, if multiple screenshots are generated:
> If multiple timemarks are passed and no variable format token ('%s' or '%i') is specified in the filename pattern, _%i will be added automatically.

Also, this comment from the code seems to convey the intention that a numbered suffix should only be used for multiple screenshots (from lib/recipes.js):
> // Add '_%i' to pattern when requesting multiple screenshots and no variable token is present

But with this code a suffix was appended although there is only one timestamp:

```
.screenshot({                                                                  
  timestamps: ['50%'],                                                         
  folder: path.dirname(output),                                                
  filename: path.basename(output),                                             
  size: '320x?',                                                               
});
```

This PR fixes this by changing a check in fixPatterns in `lib/recipes.js` and adds a test.

Note: I had 5 failing tests before and after the change, so I'm not 100% I didn't break anything else. Not sure why. Also, it seems the tests regarding the screenshots didn't get executed at all with `make test`, maybe because of other failing tests in processor.test.js, so I used something like `mocha ... --grep 'a single screenshot'` to verify that the new test fails without the change and passes after the change in lib/recipes.js.

Oh, one more thing: Awesome project. :clap: